1. Field of the Invention
The present invention relates to a developing device for a copier, facsimile apparatus printer or similar image forming apparatus and, more particularly, to an improvement in the agitation and charging of a developer in a rotary developing device having a developing unit in which a plurality of developing chambers are arranged around an axis of rotation, and rotating the unit about the axis to locate any one of the chambers at a developing position where an image carrier is located.
2. Discussion of the Background
A rotary developing device, or revolver as generally referred to, is disclosed in Japanese latent Laid-Open Publication Nos. 62-251772 and 63-78170 by way of example. The revolver is made up of a rotary developing unit, a rotary developer storing unit, and conveying means. The developing unit has a plurality of developing sections or chambers arranged around an axis of rotation. A plurality of storing chambers are defined in the storing unit in one-to-one correspondence with the developing chambers. The conveying means conveys a developer from the respective storing chamber to the associated developing chamber. The developing unit, storing unit and conveying means are rotated about the axis integrally to bring one of the developing chambers to a developing position where an image carrier is located. This developing device will be referred to as Prior Art 1.
Japanese Patent Publication No. 64-8330 and Japanese Patent Laid-Open Publication No. 61-243467, for example, teach a revolver in which the developing unit and the developer storing unit are configured independently of each other, and only the developing unit is rotatable. In this revolver, the conveying means for conveying the developer from the storing unit has its end implemented as a structural body unrotatably disposed in a bore formed at the center of the developing unit. The developer is dropped from an outlet formed in the bottom of the structural body into a developing chamber by gravity. This developing device will be referred to as Prior Art 2.
Assume that any one of the above revolvers is loaded with developers of different colors in order to form a full-color image. Then, the developing chambers of the developing unit must be sequentially replaced during the course of image formation. This brings about a problem that the developer in each developing chamber cannot be sufficiently agitated, and a problem that the charge deposited on the developer fed from the storing chamber to the developing chamber is short. The short agitation results in the irregular density distribution of an image while the short charge contaminates the background of an image and causes the developer to fly about.
To eliminate these problems, previously mentioned Publication No. 64-8330 proposes to subdivide each developing chamber into two compartments by a partition. An agitating screw is disposed in each of the compartments. An opening is formed in opposite ends of the partition to provide communication between the compartments. The screws in the compartments are driven to circulate a developer through the compartments via the openings of the partition. The developer dropped from the outlet of the structural body is received in one of the compartments and mixed and agitated with the existing developer to a certain degree. The resulting mixture is transferred to the other compartment where a developing roller is positioned. The above document also proposes to move the developer, fed from the outlet into the chamber located just below the outlet, to a developing position different from such a position and where an image carrier is located. This kind of scheme allows the replenished developer to be mixed with the existing developer and charged to a certain degree before it arrives at the developing position. Hence, even immediately after the replenishment, it is possible to effect development at the developing position without irregularity.
Laid-Open Publication No. 61-243467 also mentioned previously proposes to mount on the apparatus body an exclusive developing roller driveline which faces the chamber brought to the above-stated position just below the outlet. The exclusive driveline drives the developing roller of the chamber in order to agitate the replenished developer together with the existing developer within the chamber.
In the structure taught in Publication No. 64-8330, when the relation between the two compartments is inverted in the up-and-down direction due to the rotation of the developing unit, the developer is agitated in the vertical direction during the movement from the upper compartment to the lower compartment. However, because the vertical agitation occurs via the openings at opposite ends of the partition with respect to the widthwise direction, irregular development is apt to occur in the widthwise direction unless the developer is sufficiently agitated in the above direction, i.e., horizontal direction afterwards. Assume that the developer dropped from the outlet of the structural body is directly replenished into one compartment and mixed with the existing developer by the screw, as also taught in the above document. Then, it is likely that the charge deposited on the replenished developer is short because of the agitation efficiency lower than in the case the developer is agitated in a tube or similar limited space. Further, assume that the developer replenished into the chamber just below the outlet is moved to the developing position remote from such a position, as further taught in the above document. This configuration, however, needs a special structure as discussed in relation to Prior Art 2 and does not give any consideration to the application to Prior Art 1 having a different basic structure.
In the scheme for agitating the replenished developer in the chamber located just below the outlet, as taught in previously mentioned Laid-Open Publication No. 61-243467, the exclusive driveline is mounted on the apparatus body This increases the number of positions where the developing unit and the drive system on the apparatus body engage and disengage (luring the rotation of the revolver, aggravating the vibration of the revolver. Another problem with this scheme is that sufficient agitation is not attainable, depending on the rotation speed of the developing roller driven by the exclusive driveline.